Fire-resistant substance and method for producing a fire-resistant lining

ABSTRACT

The invention relates to a fire resistant substance and to a method for producing a fire-resistant lining. The fire-resistant substance contains MgO sinter and up to 5 wt. % of a reducing agent selected from the group consisting of substances that contain carbon. The lining has several layers with decreasing quantities of reducing agents.

The present invention relates to a method of producing a refractorylining.

Hearths of electric furnaces are frequently lined using dry hearthmasses. In addition to masses based on sintered dolomite, masses basedon MgO sinter are predominantly used.

In the related art, the grain size distribution is considered ofessential significance in the processing of the mass and itscompressibility.

Upon the (oxidative) start up of an furnace which was produced using anew mass, ceramic binding of the mass rapidly occurs. The lining istherefore as resistant as possible as soon as, for example, scrap isloaded.

European Patent 0 214 882 B1 describes a fireproof furnace lining, whichis constructed from multiple layers, the layer facing the molten metalto completely sinter during operation, while the layers lying behind it,in particular the layer facing the furnace wall, to sinter partially atmost, in order to make them easier to break out after the lining wearsout.

The fireproof mass comprises fireproof inorganic particles which areembedded in a binding agent. The different sintering properties of theindividual layers are adjusted via the grain size of the fireproofinorganic material.

The present invention is to provide the possibility of being able to,for example, produce a furnace lining which has different sinteringbehavior over its thickness.

The method according to the present invention includes features of claim1. Embodiments are described in the subclaims.

Surprisingly, it has been shown that in principle the mass must onlyinclude two components for this purpose, namely MgO sinter and areducing agent.

The use of a binding agent is superfluous. The selection of specificgrain sizes or a specific grain size distribution is of only secondarysignificance.

It is sufficient, as described, to produce the mass exclusively from MgOsinter and reducing agent, for example elemental carbon. In addition toMgO sinter, the fireproof matrix material may contain, if necessary,other refractory oxides, such as Al₂O₃, TiO₂, Fe₂O₃, CaO or the like;fired dolomite may also be used.

The sintering behavior of the (hearth) mass may be adjusted in atargeted way via the mass proportion of the reducing agent within the(hearth) mass. Through the addition of carbon (for example as graphite,carbon black, or petroleum coke) a reduction of the dicalcium ferrite inthe MgO sinter occurs and CaO and FeO arise. FeO diffuses into thepericlase (MgO sinter) and forms magnesiowustite.

By increasing the added carbon, the sintering behavior may be reducedcontinuously.

Adding pure metals such as Si or Al also encourages the reductionprocesses described, in that oxygen is consumed and the sintering of themass is delayed. Other reducing agent, particularly other C carriers,such as resins (for example, phenol resin), pitch, or sugar, are atleast partially possible.

In principle, those additives are usable which act as reducing agents,if they and/or the reaction products resulting therefrom do not lead toeffects which oppose the object of the invention. Such disadvantageouseffects would be the encouragement of the sintering of the mass or thereduction of its fire resistance (refractoring) (for example, because ofmelt phase formation). For example, sodium sulfite as a reducing agentwould lead to undesirable sodium oxide as a decomposition product.

The following table shows the cold compressive strengths after areducing firing at 1,600° C. in N/mm² for hearth masses of the typedescribed above made of MgO sinter (grain size: up to 8 mm) anddifferent carbon contents, in the form of graphite, after an oxidizingpre-firing at 1,300° C.

Cold compressive Carbon content strength 0% 46.6 1% 44.1 2% 42.4 3% 35.84% 27.9 5% 20.9

In contrast to the teaching of European Patent 0 214 882 B1, theaddition of a binding agent was intentionally dispensed with. Theeffects described arise independently of the grain size selected for theMgO sinter. This may even be <5 mm. In other words, starting from oneand the same base mass (MgO sinter), hearth masses having differentsintering behavior may be produced exclusively by selecting and settingthe quantity of the reducing agent added.

In this way, a fireproof lining may be produced in multiple layers,subsequent masses of the type described above being used, however, eachhaving a reduced proportion of reducing agent. While the first layer,for example, comprises a mass having 5 weight-percent carbon, the carboncontent of the layer subsequently applied to this first layer and/orfollowing layers is reduced to, for example, 0.5% in the last layerapplied, which neighbors the molten metal in application.

While the carbon of this last applied layer having the lowest carboncontent burns out a few centimeters deep as the furnace is put intooperation (on the fire side), so that in this layer almost completesintering is achieved, as in typical hearth masses, the degree ofsintering of the layers lying behind it is reduced more and more (withincreasing carbon content), so that, for example, the external layer,which neighbors the metal casing of the furnace, remains brittle evenafter a long period of use and is only slightly sintered, whichsignificantly favors breaking out the lining in case of repair orreplacement.

Different types of MgO sinter may be used. Due to the chemism described,MgO sinters having higher iron contents (chemical analysis), which untilnow could only be used in a limited way, are particularly suitable.

According to one embodiment, MgO sinter having a Fe₂O₃ content>1.5 or >3weight-percent is suggested, and according to a further embodiment, MgOsinter having a Fe₂O₃ content>5 weight-percent is suggested.

Expressed as a mineral paragenesis, this corresponds to a.C₂F content ofapproximately 2 or 4 weight-percent and/or 7 weight-percent.

The carbon may be added as graphite, carbon black, or the like. In anycase, a homogeneous mixture with the sintered grain is desirable. Thegrain size of the carbon may be <200 μm, and according to one embodiment<100 μm.

What is claimed is:
 1. A method of producing a refractory lining of ametallurgical melting vessel in multiple superimposed layers, whereinrefractor masses, which have MgO sinter and up to 5 weight-percent of atleast one reducing agent selected from the group consisting of metalsand carbonaceous substances, being applied sequentially, with a reducedcontent of reducing agent in each layer.
 2. The method according toclaim 1, wherein elemental carbon is used as the reducing agent.
 3. Themethod according to claim 1, wherein the reducing agent is selected fromthe group consisting of resins, pitch and tar.
 4. The method accordingto claim 1, wherein the refractory mass comprises exclusively MgO sinterand reducing agent.
 5. The method according to claim 1, wherein therefractory mass comprises MgO sinter which has a Fe₂O₃ content>1.5weight-percent.
 6. The method according to claim 1, wherein therefractory mass comprises MgO sinter which has a Fe₂O₃ content of >3weight-percent.
 7. The method according to claim 1, wherein therefractory mass comprises MgO sinter which has a Fe₂O₃ content of >5weight-percent.